In a particular application of the invention, as in the prior art, an antenna is mounted outside a building, and is motor-operated to assume a position corresponding to the setting of an electrical control. In a variety of servo systems for that purpose and others, the motor is reversible and operates an adjustable resistor representing the position of the antenna (or other device). The motor operates in one direction or the other as needed to restore bridge balance, and supply of power to the motor is interrupted when the output of the bridge indicates that the desired antenna position has been attained. Self-balancing direct-current bridges are well known, and have been used in antenna rotator systems. Several known systems are described, and a new version is disclosed in Deming, U.S. Pat. No. 3,667,024. This patent is incorporated herein by reference to the extent pertinent.
Several problems remain, however. In the patented device, two relays are used, one to control input power for the system, and the other to control the direction of the motor. The bridge and associated circuitry are so constructed that when the bridge reaches its balance point, ie., an actual null, the power control relay drops out and the system is de-energized. The power control relay must therefore reliably drop out before operation of directional relay (which either drops out or picks up, depending on the sense of the error being corrected) or the system may hunt, resulting in "flag waving" of the antenna. Deming solves this problem by specifying that the power control relay have a high dropout voltage relative to its pull-in voltage, and that the directional relay have a considerably lower dropout voltage compared to its pull-in voltage. Such relays can be expensive, which is a competitive disadvantage. Also, to the extent that aging, contamination or other factors reduce the dropout voltage of the power relay, the reliability in avoiding flag waving is degraded. Further, because the system is critically dependent on the electromechanical characteristics of the power relay as it responds to a bridge null, even if flag waving does not occur, there can be rather unpredictable accuracy variations as time passes.
Considerations such as described above have led to further developments, such as described in U.S. Patent application Ser. No. 467,883, filed May 8, 1974 to William Hohman, commonly assigned herewith. In that application, there is disclosed a system which utilizes a direct-current bridge circuit and responds not to the attainment of a null as has been common in the past, but to a "zero-crossing", that is, to a reversal of the polarity or sense of the error between the actual and desired positions of the mechanical system under control. Thus, rather than attempting to make the system come to rest in response to a given error, the system is brought to rest when the magnitude of the error starts to increase again.
In the Hohman device, the zero crossing itself is not sensed, but rather the rate of change of a signal representing position error. For this purpose, the DC bridge is coupled to a high gain differential amplifier operated "open loop", ie. having no degenerative or stabilizing feed-back. The amplifier provides a large output signal of one polarity virtually up to the point of true null and then reverses abruptly, to provide a large signal to opposite polarity (in relation to a mean potential of the system) as the sense of the error changes, and the magnitude begins to increase. The amplifier output is differentiated, and is utilized to interrupt operation of the motor.
In Hohman, one sense of reversal of the amplifier output is utilized only to reverse the motor. The next-following operation of the motor produces a motor-stopping sense of reversal of the amplifier output as null is again traversed, and the system coasts to rest after a limited angle of overtravel. With that form of control, the system always comes to rest after passing null in the same direction. Exacting standards of accuracy can be attained but the drive motor, with the inertia of the driven load, must be stopped and then reversed in direction in about half of the operations. The resulting increased "use" can affect the durability of the system, and must be taken into account in component design.